The International Atomic Energy Agency (IAEA), the United Nations nuclear watchdog, has found trace amounts of uranium in an Iranian warehouse, Reuters reports. The leftover uranium has yet to be explained by Iran, according to diplomats privy to the inspection cited by Reuters. The site where the traces were found is located in Tehran, Iran’s capital city, and has been called a “secret atomic warehouse” by Israeli Prime Minister Benjamin Netanyahu. Iran’s lack of explanation for the particles fuels tensions between Iran and the West, notably the United States. Iran broke an international nuclear deal in May in response to U.S. sanctions from the Trump administration.

That breach in part led to Netanyahu’s call for the IAEA to visit the long suspected warehouse. After inspecting the site, the watchdog reportedly found traces of radioactive matter. The IAEA has now determined that those traces were in fact uranium, according to the diplomats, Reuters reports. One of the diplomats reportedly said the samples were not enriched, and therefore were not viable for atomic weapons.